The most generally used cooling fluid is water in the temperature range above the freezing point. In addition to its ready availability, the advantage of water is that all liquids it has the highest specific heat. it has low viscosity and, as a result of its physical properties it has good heat conductivity. In addition to these important thermal characteristics, it has a relatively moderate corrosive effect, which can be further reduced by the addition of inhibitors.
However, the disadvantage of water is that it is not frost-resistant. Upon freezing its volume increases by 9%, leading to the frost damage of equipment.
As used throughout the specification and the claims, characterizing terms such as "frost free" or "resistant to freezing" means that a material so characterized does not freeze at least substantially below the freezing point of water. Also as used throughout the specification and claims, terms such as "coolant" and "cooling fluid" are used interchangeably with the same meaning.
The frost resistance of water can be significantly improved by the addition of electrolytes that stay dissolved even at low temperatures. This possibility is, however, very much limited by the fact that the use of acids and bases is not permissible in many places for industrial safety, environmental protection and due to their corrosive nature. From a cryoscopic aspect calcium chloride has been proved to be most advantageous, because by its addition a temperature of even about 50.degree. C. can be attained, while the solution maintains its liquid state, and only a slight increase in viscosity and a slight reduction in specific heat takes place. A much more serious problem, however, is the corrosion activating effect of the chloride ions, mainly in concentrated solutions. This effect which causes mostly a perforation corrosion, cannot be satisfactorily reduced even by the use of any known corrosion inhibitors. Solubility problems arise if other anions are used instead of chloride. When e.g. calcium sulfate is used, these problems can involve a low saturation concentration under all conditions, or when, for example, sodium sulfate or potassium nitrate is used, a dramatic decrease of saturation concentration will occur with decreasing temperature.
Hydrocarbons, alcohols, and ketones of a low carbon atom number are very advantageous with respect to frost resistance, however, the low specific heat, high volatility, i.e. low boiling point as well as the risk of fire and explosion all counsel against their use; all the more so, because certain members of this group of organic compounds, such as methanol are also poisonous, toxic and are therefore, unsafe to use in an industrial environment.
Due to their various advantages, the aqueous ethylene glycol solutions marketed as heat transfer liquids under various trade names have been accepted in a wide range as frost resistant coolants. The advantage of these solutions is that glycol can be mixed with water without limitations, fire and explosion risk is moderate, the specific heat is about twice as high as that of the aforementioned organic solvents, they are colorless, odorless and relatively frost resistant. Specific heat and solidification point are functions of the glycol-water ratio. With the increase of water content the specific heat increases, but frost resistance decreases. The lowest solidification point (-57.degree. C.) is associated with the eutectic composition. In this case the glycol content is 60% by volume. A further advantage is that in the practically important compositions the glycol-water mixture does not freeze with a concomitant increase in volume. Consequently apparatus cooled with a glycol-water coolant will not be freezing.
There are also disadvantages which limit the wider use of glycol based coolants. Ethylene glycol is very toxic and it can be easily confused with ethyl alcohol. The ingestion of ethylene glycol has led repeatedly to well publicized, occasionally fatal poisoning cases. Ethylene glycol can also contaminate the environment, and grave consequences as a result if it gets into bodies of water, such as when the coolant is carelessly discarded or when it leaks from defective equipment. The frost resistance of ethylene glycol can be only partially utilized because for example an aqueous mixture of 1:1 volume ratio has a freezing point of -38.degree. C., but at -31.degree. C. it already shows a jelly-like nonthixotropic consistence, and is thus unsuitable for pumping. The so called fluidity limit of -31.degree. C. cannot be further reduced even by modifying the glycol: water ratio, and this also means that below -30.degree. C. in complications, such as heat transfer processes were low viscosity is required the glycol-water mixture cannot be employed. The frost resistance and fluidity decrease, and the corrosiveness of aqueous ethylene glycol increases in time, due to slow polymerization and acidic decomposition of the glycol. Therefore, the mixture has to be replaced from time to time, and this increases operating expenses.